A new Schiff base-fabricated pencil lead electrode for the efficient detection of copper, lead, and cadmium ions in aqueous media

Cu2+, Pb2+, and Cd2+ were individually and simultaneously determined using a novel and effective electroanalytical approach that has been devised and improved. Cyclic voltammetry was used to examine the electrochemical properties of the selected metals, and their individual and combined concentrations were determined by square wave voltammetry (SWV) using a modified pencil lead (PL) working electrode functionalized with a freshly synthesized Schiff base, 4-((2-hydroxy-5-((4-nitrophenyl)diazenyl)benzylidene)amino)benzoic acid (HDBA). In a buffer solution of 0.1 M tris–HCl, heavy metal concentrations were determined. To improve the experimental circumstances for determination, scan rate, pH, and their interactions with current were studied. At some concentration levels, the calibration graphs for the chosen metals were linear. The concentration of each metal was altered while the others remained unchanged for both the individual and simultaneous determination of these metals, and the devised approach was proven to be accurate, selective, and rapid.


Introduction
Toxic heavy metals are released into the environment in everincreasing quantities, endangering humanity as they build up in the soil and in human bodies due to their inability to disintegrate. They also pose a serious issue as pollutants in waste that is dumped into outows. The cumulative properties and prolonged biological half-lives of these heavy metals make them potentially harmful to human health when exposed to them. Reactive oxygen species (ROS), in particular polyunsaturated lipids and proteins, can disrupt cell macromolecules as a result of metal toxicity. 1 It is therefore important to detect and quantify heavy metals in soil and streams for health reasons. 2 Spectroscopy, 3 optical colorimetry, 4 inductively coupled plasma 5 mass spectrometry (ICP-MS), 6 atomic absorption spectrometry (AAS), 7 and uorescence spectrometry 8 are some of the analytical methods used today for heavy metal detection. However, they are expensive (they usually need expert operators and sophisticated equipment), therefore, electrochemical approaches have been researched as alternatives 9 due to the ease of sample preparation and simpler procedures. In addition, electrochemical techniques for heavy metals analysis have certain advantages owing to their high sensitivity, precision, simplicity, and low cost. 10 Schiff bases are chemical compounds formed when a primary amine reacts with an aldehyde or a ketone to form an imine. 11 Schiff bases can be easily altered by selecting the right amine and substituting a carbonyl molecule, resulting in a wide range of structural diversity. Schiff bases are widely used in electrochemical applications owing to their characteristic structure and their ability to bind to electrode surfaces. [12][13][14] The search for an optimum pencil graphite electrode (PGE) for the detection of trace heavy metals has exploded recently. Working electrodes made of carbon-based materials are oen employed in electrochemical analysis 15 owing to the ease with which they can be used to pre-treat surfaces. Given their convenience, time-saving nature, and compliance with green chemistry standards, disposable pencil-graphite electrodes (PGEs) are considered an excellent choice as working electrodes for both laboratory and on-site screening experiments. Pencil-graphite electrodes that have undergone electrochemical treatment also offer benets for being small, affordable, easy to create, and simple to use. The graphite-based working electrodes are widely used in electrochemical analysis owing to its abundant porosity, high electrical conductivity, and large specic surface area. 16,17 They also support rapid mass transport and fast electron transfer in electrochemical applications. It is crucial to be able to change the graphite in PGEs in just one step 6 and this depends on the supramolecular interaction of the ions with a functional group on the electrode surface, which can allow Schiff base-modied electrodes to be able to acquire the target metal ions, and which has thus piqued research interest in recent years. 18 In this study, we tried to achieve sensitive metal sensing by modifying a very cheap pencil lead (PL) electrode with the electrochemically modied Schiff base 4-((2-hydroxy-5-((4-nitrophenyl) diazenyl)benzylidene)amino)benzoic acid (HDBA), and termed this the HDBA/PL electrode. For this reason, we applied HDBA lm to the PL electrode and performed numerous measurements. We investigated the PL electrode's surface area, the effects of pH, the voltammetry scan rate, and calculated the diffusion coefficient using cyclic voltammetry (CV). Additionally, individual metal detection by square wave voltammetry (SWV) was performed based on changes in the current from varying the concentration of specic metals, as well as the simultaneous determination of multiple metal ions, also by (SWV). 19

Chemicals and apparatus
The chemicals used in this study included 4-amino benzoic acid, tris-HCl buffer, methanol, dichloromethane, glacial acetic acid, CuSO 4 $5H 2 O, Pb(NO 3 ) 2 , and Cd(NO 3 ) 2 $4H 2 O. A Gamry REF 3000 potentiostat/galvanostat/Zra and Echem Analyst 7.8.2 program for data processing, tting, and graphing were used for all electrochemical studies. The cell conguration used was a typical three-electrode conguration, with the HDBA-modied PL electrode utilized as the working electrode, and platinum wire and saturated Ag/AgCl utilized as the counter and reference electrodes, respectively. 20,21 . Bruker DMX-400 spectrometers running at 400 and 101 MHz were used to measure the 1 H and 13 C NMR spectra. An Agilent Cary 630 spectrometer was utilized to measure the FT-IR spectra. A 3520 pH meter was utilized to measure the pH levels (Jenway, UK). A Stuart melting point SMP30 instrument was used to measure the melting points.

Synthesis of HDBA
First, 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde (200 mmol) 22 and 4-amino benzoic acid (200 mmol) were reuxed in ethanol for 3 h to afford a new Schiff base (HDBA), which was washed, dried, and recrystallized from ethanol to obtain a highquality product in a 60-65% yield with a melting point of 250°C (Fig. 1). IR, 1 H-NMR, and 13 C-NMR spectroscopy were used to determine the chemical structure of the synthesized Schiff base, as illustrated in Fig. 2, 3, and 4, respectively. 20 The IR spectrum of HDBA displayed absorption bands at

Construction of the HDBA/PL electrode
PL with a diameter of 0.9 mm and a length of 6 cm was used to create the working electrode. The surface wax was rst removed using white print paper, and 1 cm of the PL was then dipped into a solution containing DCM and 1 : 1 HDBA + PVC to electrochemically synthesize HDBA lm on the PL. To remove any potential impurities, the as-fabricated HDBA/PL electrode was soaked in 0.1 M tris-HCl solution for 15 min before being placed in a desiccator at room temperature for further testing. 19

Electrochemical measurements
Utilizing cyclic voltammetry (CV) and square wave voltammetry (SWV), electrochemical tests were conducted. In 0.1 M tris-HCl, the potential was examined by CV analysis. The SWV parameters were: frequency of 12.5 Hz, 50 ms pulse width, 2 mV scan increment, and 50 mV pulse height. First, 200 ppm metal ions in solutions were generated at pH 2-10. Then, the relation between the potential and current was established and the pH was selected with the largest current that would impact the scan rate at a concentration of 50 ppm (CV). The relation among the    The 9.13 ppm singlet corresponded to the proton on the imine group, while the 10.32 ppm singlet was attributed to the proton on the phenolic group. Finally, the proton on the carboxylic acid group was indicated by the signal at 13.55 ppm.
The 13 C-NMR spectrum of HDBA ( Fig. 4) displayed several signals that were attributed to carbon atoms, notably the carbon atoms in the benzene ring, with signals between 120 and 140 ppm, in the azo group at 165.42 ppm, the C]N group at 157.12 ppm, and the carboxylic group at 186.66 ppm. These results were consistent with the expected chemical structure of the synthesized Schiff base.

Electrochemical measurements
3.2.1. Electrochemical characterization of the HDBA/PL electrode using the standard potassium ferricyanide system. The as-fabricated HDBA/PL electrode was electrochemically characterized using CV and a 10 mM ferricyanide redox probe dissolved in 0.1 M KCl. The ndings showed that the addition of the Schiff base HDBA to the PL electrode increased the peak currents ( Fig. 5a and b). The signal amplication was outstanding and was related to the increase in the electrode's electroactive surface area. 23 To calculate the HDBA/PL electrode effective surface area for a reversible reaction, the Randles-Sevcik equation, was utilized: 24,25 where, i p is the current maximum (Ampere), the constant 2.69 × 10 5 has units of C mol −1 V −1/2 , n denotes the number of transported electrons, A is the effective surface area of the HDBA/PL electrode, which was 0.06 mm 2 , and D, C, and v are the diffusion coefficient (cm 2 s −1 ), the bulk concentration of the redox probe (mol cm −3 ), and the voltage sweep (V s −1 ), respectively. For K 3 Fe(CN) 6 , n = 1 and D = 7.6 × 10 −5 cm 2 s −1 . Here, a linear association was found between the peak current vs. square root in the scan rate plot, indicating that the electrochemical kinetics was a diffusion-controlled reaction (Fig. 5c).
With a correlation value of R 2 = 0.9999, the slope of the plot was calculated to be 56.133. According to the ndings, the HDBA/PL electrode is suitable for the electroanalysis of heavy metal ions. 26 Next, electrochemical impedance spectroscopy (EIS) tests were done for the bare and HDBA electrodes in 10 mM ferricyanide redox probe dissolved in 0.1 M KCl in the frequency range of 0.1 Hz to 10 5 Hz. The Nyquist spectra of the bare and HDBA/PL electrode are shown in Fig. 6. The electron transfer resistance R CT was calculated from the Nyquist plot semicircles  by tting to the suitable equivalent circuit. 27,28 The modied electrode had a smaller diameter, lower R CT , and higher conductivity than the bare electrode, whereby the value of R CT for the bare electrode was 12.440 kU and the value of R CT for the HDBA/PL electrode was 0.698 kU. Also, the bigger semicircle of the bare electrode demonstrated its greater interface impedance. 29,30 The spectra were in good agreement with the outcomes of the CV experiments shown in Fig. 5a, demonstrating the modied electrode's increased conductivity. 31,32 3.2.2. Effect of pH. In 0.1 M tris-HCl buffer, the effect of Cu, Pb, and Cd(II) on the maximum current was investigated. 33 The results are given in Fig. 7a-c, showing the ideal pH for copper, lead, and cadmium determination was 6, 2, and 5, respectively. The height of the Cu, Pb, and Cd(II) peak currents dropped when the acidity was higher or lower than the ideal pH value. 34 The linear graphs of the peak potential's pH dependency are shown in Fig. 7d. 3.2.3. Impact of the scan rate. The relation between the peak current and scan rate can allow estimating important electrochemical mechanism information. 33 Therefore, CV was applied to determine how the scan rate affected the peak potential (E p ) and peak current (I p ), as shown in Fig. 8a, 9a, and 10a. It is known that E p is independent of v and vice versa if the electrooxidation reaction is reversible. 35 Fig. 8a, 9a, and 10a show that as the scan rate increased, the anodic peak potential (E p ) moved to a higher potential, suggesting that the electron transfer in the analyte electrooxidation was irreversible. According to Fig. 8a, 9a, and 10a, the anodic peak current (I pa ) increased with the scan rate increasing from 0.02 to 0.2 V s −1 , indicating that the electron transfer reaction took place. 36 A reasonable estimate of the surface concentration of the electroactive species (G) can be made using the slope of the linear plot of I versus v in Fig. 8b, 9b, and 10b by using the following equation: 37 A linear relationship between log I p and log v was found (Fig. 8c, 9c, and 10c). For the current regulated only by diffusion, the slopes of 0.5654, 0.3729, and 0.3862 V s −1 in the gures were close to the value of 0.5 V s −1 . Consequently, the I versus v 1/ 2 curve was linear, most likely as a result of diffusion-controlled oxidation (Fig. 8d, 9d, and 10d). According to Laviron, the following equation denes E p as an irreversible electrode process: where E°denotes the formal oxidation and reduction potential, n denotes the number of transported electrons, a denotes the transfer coefficient, k°denotes the reaction's standard heterogeneous rate constant, and v denotes the scan rate. The computed (k°) values for Cu, Pb, and Cd at T = 298 K, R = 8.314 J K −1 mol −1 , and F = 96 480 C mol −1 were 2596, 2450, and 3760, respectively (Table 1). 38 The plots of E of the analytes versus log v are shown in Fig. 8e, 9e, and 10e. The Bard and Faulkner equation 39 was used to get the value of a: where E p/2 is the potential where the maximum current is cut in half, and based on these values of a was equal to 0.75, 0.81, and 0.9 for Cu, Pb, and Cd, respectively. Also, n for Cu, Pb, and Cd were also estimated to be 1.76-2, 1.78-2 and, 1.92-2, respectively (Table 1). 38,40 According to Table 2, the peak potential favorably moved from 0.356 to 0.367 V when the scan rate was increased from 20 to 200 mV s −1 as a result of diminishing the diffusion layer growth in the electrode, 41 and consequently, the estimated diffusion coefficient, D, increased linearly with the scan rate variation.
At a potential of approximately −0.09 V, the anodic behavior of Cu was visible, with a sensitivity of 1.9214 (Table 3). Fig. 12a illustrates the results for the determination of Pb(II) at the modied electrode. According to the amount of Pb, the peak current increased gradually when the potential was about −0.49 V, where the anodic peaks could be observed. 39 The SWV responses of the modied HDBA/PL electrode to Pb(II) in the concentration range of 7.5 to 24.57 ppm (Table 3) are shown in The sensitivity was 2.7689. Fig. 13a shows the SWV responses of the modied electrode to Cd(II). The following is the linear equation for Cd(II) at a concentration of 7.5 to 24.57 ppm (Fig. 12b): The sensitivity was 6.1955 (Table 3). 43 3.2.5. Simultaneous determination of the heavy meal ions 3.2.5.1 Simultaneous determination of copper and lead. To determine whether the presence of multiple cations has an effect, we tested the simultaneous detection of Cu 2+ and Pb 2+ in the same solution. First, the Cu 2+ concentrations were set with varying the Pb 2+ concentration, and vice versa 44 (Fig. 14a and b). Lead's effect on 14.89 ppm copper in 0.1 M tris-HCl buffer was studied (Fig. 14a). The peak currents of Cu(II) were slightly enhanced by the increasing lead concentration (14.89,19. The HDBA/PL electrode had a lower sensitivity to Pb(II) when copper was present as 2.5212, and a higher sensitivity without copper of 2.77. The oxidation of Pb-Cu intermetallic compounds could be noted by the shoulder peaks near Pb. Furthermore, the peak currents of copper marginally decreased from −0.8 to −0.17 V. The impact of copper on 14.89 ppm lead in 0.1 M tris-HCl buffer was next studied (Fig. 14b). The peak currents of Pb(II) were somewhat reduced with increasing the copper concentration (14.89, 19.75, and 24.57 ppm), while the two peaks at −0.41 and −0.48 V signied lead's two-step oxidation. The linear equation for Cu(II) in the concentration range of 14.89 to 24.57 ppm was as follows: The HDBA/PL electrode had a higher sensitivity to Cu(II) when lead was present (2.0413) than for copper without lead (1.9214), because Cu depressed the Pb wave and caused the formation of Cu-Pb intermetallic complexes, and so the sensitivity for copper increased while the sensitivity for lead decreased. Cu's sensitivity was increased by the oxidation of the Cu-Pb intermetallic complexes, which took place close to Cu's square potential (II).
3.2.5.2 Simultaneous determination of copper and cadmium. Cadmium's effect on 14.89 ppm copper in 0.1 M tris-HCl buffer  was investigated. 45 The peak currents for Cu(II) were slightly increased as the cadmium concentration increased (14.89, 19.75 When copper was present, the HDBA/PL electrode had a lower sensitivity to Cd(II). The sensitivity to cadmium without copper was 6.1955. The emergence of a wide peak in between the two main peaks (Fig. 15a) might be caused by hydrogen evolution brought on by the catalytic action; on copper electrodes, it has been demonstrated that this can happen at potentials as high as 0.40 V. The shoulder peaks around Cd could be attributed to the oxidation of Cd-Cu intermetallic complexes. Also the change in the copper peaks to more negative values was also notable. Such a peak shi is a type change of the deposited phase's signature and a marker of evolution in the Cd-Cu phase diagram. Next, the impact of copper on 14.89 ppm of cadmium in 0.1 M tris-HCl buffer was investigated. Almost no Cd(II) stripping peaks were visible. This should be attributed to the formation of a Cd lm on the surface of the modied electrode during the deposition process, since the electrode surface has a large affinity for Cd(II), which raises Cu's sensitivity. The following is the linear equation for Cu(II) with regard to concentrations of 14.89 to 24.57 ppm: 26 When cadmium was present, the HDBA/PL electrode showed lower a sensitivity to Cu(II) of 0.8568, whereas copper's sensitivity in a cadmium-free environment was 1.9214. The oxidation of Cu-Cd intermetallic complexes was responsible for the shoulder peaks close to Cu, while the emergence of a wide peak (Fig. 15b) in between the two main peaks might be caused by hydrogen evolution brought on by copper's catalytic activity.
3.2.5.3 Simultaneous determination of lead and cadmium. Next, we investigated the impact of cadmium on 14.89 ppm lead in 0.1 M tris-HCl buffer. Varying the amounts of cadmium When there was lead present, the HDBA/PL electrode showed a lower sensitivity to Cd(II). The sensitivity of cadmium without lead was 6.1955 versus 0.9738 with lead present. We investigated the impact of lead on 0.1 M tris-HCl buffer containing 14.89 ppm cadmium, with lead concentrations of 14.89, 19.75, and 24.57 ppm (Fig. 16b) The HDBA/PL electrode had a greater Pb(II) sensitivity when cadmium was present (5.7362), compared to without cadmium (2.7689). The cadmium peak was tilted more in the direction of negative values. Peak shiing is a type of change in the deposited phase's hallmark that denotes a progression in the Cd-Cu phase diagram. The effect of Cd on Pb(II) sensitivity was essentially nonexistent. Since there were no shoulder peaks, it  could be concluded that the peak suppression of Cd by Pb was not caused by the intermetallic interference between Cd and Pb. 46 Rather, the increased diffusivity and higher standard reduction potential of Pb(II) may be the cause.
3.2.6. Selectivity of the HDBA/PL electrode. Under pHoptimized circumstances, interference analysis was carried out by detecting 34.23 ppm Cu(II), 5 ppm Pb(II), and 25 ppm Cd(II) in the presence of 20-fold concentrations of interfering ions (Fig. 17-19). The peak currents of Cu(II), Pb(II), and Cd(II) were oen changed by less than 10%, as shown in Table 4, while the peak currents of Cu(II) increased by 12.2% with Na 1+ and the peak current of Pb(II) decreased by 15.9% with Ca 2+ . It is likely that the excessive Na 1+ , Ca 2+ , and target metals on the electrode surface may be competing with one another, which might be the cause for these results. In the SWV tests, the HDBA/PL electrode showed good selectivity for Cu(II), Pb(II), and Cd(II). 47,48

Conclusion
This article looked into the specic electrochemical behavior of Cu, Pb, and Cd using a simple HDBA/PL electrode. The described modied electrode greatly enhanced the electrochemistry of the metals and amply illustrated the remarkable HDBA/PL electrode's activity in terms of the anodic response to heavy metals. The electrochemical properties were computed for the apparent charge transfer rate constant, transfer coefficient, diffusion coefficient, and surface concentration of the electroactive species. Satisfactory analytical performance was obtained under the optimized experimental circumstances, including an adequate precision. The technique was efficient enough to analyze heavy metal contents at lower levels. Additionally, the suggested approach does not call for pricey equipment or essential analytical reagents.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.